This invention relates to optimizing properties of polymeric resins, particularly polyolefins, made in continuous production runs, including transitions from the manufacture of one polymer product to another. It includes methods and apparatus for utilizing maximum amounts of products having properties which vary within and/or outside a set of specifications.
In many polymer manufacturing processes, off-grade resin may be produced when transitioning from one resin grade to another and even during steady state operation. This problem is especially noticeable in a gas phase polymerization because the large fluidized bed polymerization reactor behaves like a continuously stirred tank reactor. Once the reactor conditions are changed for the new grade of resin in a gas phase reactor, it may be necessary to produce as much as three times the bed weight of the resin before the new resin properties achieve an acceptable steady state. Because the product properties change slowly in large volume reactors, and depending on the similarities and differences between the two products, it is not uncommon for up to 15% of the total resin made in a reactor campaign to be off grade (out of specification in at least one respect), mainly due to the product made during the transition from one set of specifications to another. The resin produced during such transitions is usually sold at a significantly lower price than the resin that meets the specifications (sometimes called AIM grade resin herein). It is desirable to reduce the amount of off-grade resin made during transition periods between sets of specifications and at any other time.
In gas phase polymerization operations, it is common to pellet the off grade granular resin and then recycle these off grade pellets back into the pelleter at concentrations which will not cause deviation from specifications in AIM grade production. Repeated pelleting can impart an undesirable thermal history, and the additive content of the product must be continually recalculated and adjusted. In addition, the pelleter capacity is underutilized because a significant portion of the granular product is run through the pelleter more than once. Recycling pellets to the pelleter is undesirable.
Another problem in a gas phase polymerization line is the provision of adequate surge capacity between the reactor and pelleter. Adequate surge capacity is important to allow common pelleter problems to be fixed without having to kill or upset the reactor, which normally produces product more or less continuously. Sudden kills or upsets in the reactor due to the inability of the pelleter to handle the particulate product can result in off grade resin and lengthy restart procedures. In the present state of the art, the purge bin located between the reactor and the pelleter is frequently designed larger than required for simple purging in order to allow extra surge time for the pelleter.
Also in continuous polymerization production, as in a fluidized bed, it is common for one or more properties to vary from a target point but remain within a specification range. While virtually all consumers and users of resin want product within specifications needed for their particular purposes, many also are concerned with the consistency of specified values. Thus the products must not merely be within a specified range of values for one or more properties, but must consistently, from run to run, and from the beginning to the end of a run, be on, or very near to, the target value or target values for one or more properties. Producing final products having reduced variability will increase the process capability limit (CpK) of the process. Persons skilled in the art will realize that a change in a control (set) point to adjust one product property will affect the process in other ways, and that process adjustments of any kind may affect product consistency in more than one manner. Ideally, in spite of the complexity of the process, one will want to produce a product which is consistently on target in all respects.
It would be desirable to alleviate the necessity for excessively large purge bins, to develop a method for minimizing off-grade production, to reduce or eliminate the practice of recycling through the pelleter, and to produce a specification product which is consistently on target for various properties.
Our invention, will be discussed and described with particular reference to the widely used xe2x80x9cUnipolxe2x80x9d fluidized bed process for olefin polymerization. See, for example, the descriptions in Bernier et al U.S. Pat. No. 5,453,471, Miller U.S. Pat. No. 4,003,712, Aronson U.S. Pat. No. 4,621,952, and other related patents of Union Carbide Corporation. The applicability and value of the invention is not limited to the Unipol process, however; it may be used in any resin producing facility which makes particulate (sometimes herein used interchangeably with xe2x80x9cgranularxe2x80x9d) product more or less continuously whether or not it is to be pelletized.
This invention provides a method for trimming, or blending, off grade granular material into AIM grade material while keeping the AIM grade material within specifications. The value of the invention is particularly high when trimming granular transition off grade into the leading and trailing products of a transition to reduce the total off grade product. The invention also provides a method for consistently producing resin which is not only within specification ranges but on target or very close to target for desired properties.
The apparatus used in this invention is a set of one or more bins, referred to herein as trim forward bins, located between the reactor and the pelleter, preferably after a purge bin. Resin from the reactor can be transferred to any of the trim forward bins, or be sent to a purge bin, or directly to the pelleter. Our invention, however, concerns the use of the trim bin or bins. Resin can be combined from any of the trim bins before it is sent to the pelleter. The system may comprise any number of bins and the bins may be of any size. An optimum number of bins and the size of the bins can be determined based on the anticipated product cycle of the reactor. Preferably the trim forward bins will be of a design or construction as to provide xe2x80x9cplug flowxe2x80x9d, meaning that the bin delivers granular material substantially in the sequence in which it was received. The interface between two somewhat different granular materials is substantially conical if the granular material enters at the center of the top of the bin and falls onto a bed of granular material already present. A conical interface between products of somewhat different characteristics, as between on-specification and off-specification polyethylene granules, will remain substantially conical as it descends in the bin. Gravity discharge from the center of the bottom of such a bin assures a substantially clear passage from one type of product to another, although there may be some mixing at the interface when it is first formed and as it emerges from the bottom of the bin. Sometimes xe2x80x9cplug flowxe2x80x9d in this context is used interchangeably with xe2x80x9cmass flowxe2x80x9d, and that is our intention in this description. The downward progress of an interface or boundary of two different types of granular products may be calculated and/or projected as discharge from the bin is monitored, and we utilize this fact in our invention. We do not intend to rule out the use of other types of trim bins, however, i.e. ones which may more significantly blur the boundary between one product and another, as there may be instances in the use of our invention where such a blurring will be well defined and/or of little consequence, and the progress of a blurred interface may be followed as well as a sharply delineated one.
Our process includes a method of enhancing the production of a first pelletized resin product having a first set of specifications made as granules in a substantially continuously operating reactor during a campaign including the scheduling of a second pelletized resin product having at least one specification of a value different from that of the first pelletized resin product comprising (a) optionally passing granules having the first set of specifications from the reactor directly through a conduit or through a trim bin to a pelleter, (b) collecting granules from the reactor having the first set of specifications in a trim forward bin, (c) changing the conditions of the reactor from conditions for manufacturing granules having the first set of specifications to new conditions for manufacturing granules to be pelletized as the second pelletized resin product (d) passing granules made under the new conditions in the reactor through a conduit to the pelleter and (e) blending granules from the trim forward bin into the granules made under the new conditions, in the conduit. It should be understood that, where product is not to be pelletized, that is, where it will be sold in granular form, the process is the same except that the destination is a holding bin or shipping container rather than a pelleter.
Additionally, our process contemplates controlling the ratio of blending in step (e) to optimize the utilization of granules made under the new conditions which are outside of specifications for either the original product or the new product. We also contemplate controlling the quantity of granular product collected in step (b) as a function of the anticipated production of off-grade granules between steady state production of the first and second products. And, our process also contemplates the utilization of more than one trim forward bin for the collection of granules having different properties for versatile calculation of blends. Additionally, our process contemplates the placing of granules of one set of specifications on top of granules of another set of specifications in the same trim forward bin, and monitoring the downward progress of the interface between the two quantities of granules as the bin is emptied, to facilitate the desired blending steps.
In another aspect, our process is a method of operating a resin manufacturing process to optimize the amount of product produced within specifications when converting from the production of a first specification product having a first set of specifications to the production of a second specification product having a second set of specifications wherein a reactor produces transition products which are sequentially (a) a first transition product which is blendable into the first specification product to obtain a first blended product within specifications (b) a second transition product which is not blendable into either the first or the second specification product to obtain a product within specifications, and (c) a third transition product which is blendable into the second specification product to obtain a product within specifications, the method comprising: (1) placing a quantity of the first specification product into a first trim bin, (2) blending the first particulate product from the first trim bin into the first transition product (a) as the first transition product (a) is forwarded to a pelleter, (3) placing the second transition product in either the first trim bin or a second trim bin (4) placing the third transition product in either the first trim bin, the second trim bin, or a third trim bin, and (5) blending the third transition product with the second specification product as it is forwarded to a pelleter.
In another aspect, our invention may be used in the situation where, in the above summary, the transition product (b) which is not blendable into either the first or second specification product is not made. That is, because of the peculiarities of the specifications of both products, only intermediate products (a) and (c) are made. In this case, the xe2x80x9cthird transition productsxe2x80x9d in steps (4) and (5) may be replaced by a second transition product, which is in this case blendable.
It is also possible to have only one intermediate product. For example, the first specification product may contain a small amount of a comonomer and the second specification product is otherwise similar but does not have the comonomer. So long as the intermediate product contains comonomer, it may be blendable (in various proportions depending on the monomer content) in the first specification product. In another variation, the specifications and/or product property values may be such that an intermediate product may be blended only into the second specification product.
Our invention includes a method of making a granular resin product having a target value z of a property A comprising placing a first granular product having a measured or estimated value x of property A in a trim bin and blending the first granular product from said trim bin into a conduit containing a second granular product having a known or estimated value y of property A to form a third, blended, granular product having a value z of property A. In yet another aspect, our invention is a method of producing resin which is not only within a specification range for a particular property, but consistently on target for the property or within very narrow limits for the property. That is, our invention includes a method of controlling granular resin product in substantially continuous production to enhance conformity to a single set of specifications comprising (a) first placing granular product of a second set of properties on top of granules of a first set of properties in a trim forward bin to form an interface between the quantities of granular product, (b) determining the downward progress of the interface as the bin is emptied, and (c) controlling the delivery of each of the quantities of granular products to a pelleter by itself or as a blend with granular product having a third set of properties at least partly as a function of the position of the interface in the bin.
Further, our invention includes a method of operating a resin manufacturing process to optimize the amount of product produced within specifications when converting from the production of a first particulate product having a first set of specifications to the production of a second particulate product having a second set of specifications wherein a reactor produces at least one transition product which is (a) blendable into the first particulate product to obtain a product within the first set of specifications, (b) not lendable into either the first or said second particulate product to obtain a product within specifications, or (c) blendable into the second particulate product to obtain a product within the second set of specifications, the method comprising: placing a quantity of at least one of the first particulate product, transition product (a), and transition product (c) into a trim bin and blending it into a later-produced product in a ratio calculated to obtain a final product having a desired set of specifications. The desired set of specifications may be either the first or second set of specifications, and the later-produced product may be either transition product (a), into which the first product is blended, or the second particulate product, into which transition product (c) is blended. The specifications may be a single value or a range of values for a single property.
Other aspects and variations of our invention will be further described below.